1. Field of the Invention
The present general inventive concept relates to an electrophotographic toner, and more particularly, to an electrophotographic toner having properties so as to obtain a stable fixed image and printing quality in a fixating system with a multiple pressure configuration.
2. Description of the Related Art
An image forming apparatus such as an electrophotographic device or an electrostatic recording device forms an electrostatic latent image by executing an image exposure on a photoreceptor homogenously charged, and attaches a toner on the electrostatic latent image in a toner image, and the toner image is transferred on to a transfer material such as transfer paper. Sequentially, a non-fixed toner image is fixed on the transfer material using a variety of methods such as heating, pressing, or solvent steaming. In most cases, the fixing process includes passing the transfer material, on which toner image has been transferred, between a fixing roller and a pressure roller, and fusing the toner on the transfer material by heat-pressing. Then, the fixating toner is fixed on the transfer material depending on fixing conditions to form a stable image.
The heat fixing method using a heating roller or a film requires high heat efficiency in order to fuse and attach the toner image on a fixing sheet when the heating roller or the film surface is in contact with the toner image. In case of the heat fixing method, an increase in a heat capacity of a heat fixing unit is needed in order to prevent fixture failure caused by fixation at a low temperature condition and passing of the fixing sheet. Therefore, maintaining a fixation property while achieving low power consumption greatly depends on improvement of the toner, and particularly, on a low temperature fixation property of the toner.
For example, in the pressure-heat fixing method, the high-temperature roller surface and the toner image come in contact under pressure in a fused form, and thus a portion of the toner is transferred and adhered to a surface of the fixing roller, and then, is re-transferred on a successive fixing sheet, thereby contaminating the fixing sheet. This is referred to as an offset, which is greatly affected by a temperature and a speed of fixing. Generally, the surface temperature of the fixing roller is set low when the fixing speed is low, and is set high when the fixing speed is high. This is because a constant energy is supplied to the toner image in order to fix the toner image, regardless of a difference in the fixing speed.
In order to solve the problems above, the fixing temperature was generally increased when the fixing speed was high, in order to promote fixation of the toner on the fixing sheet. According to the method, the heating roller temperature can be lowered to some extent, and high temperature offsetting of the top portion of a toner layer can be prevented. However, when a very high electrical output is applied to the toner layer, a number of difficult problems such as an easily occurring winding offset involving that the fixing sheet is wrapped around the fixing roller, and a trace of means separating the fixing sheet from the fixing roller onto the fixed image may easily occur.
In particular, in the case of a 3-roller/belt fixing system as an example of a fixing system with multiple pressure configuration, as illustrated in FIG. 1, and not a fixing system having a symmetrical single pressure distribution such as a 2-roller (1 heater-roller+1 pressure-roller) system, a melting strength critically affects a quality of the printing.
In this case, the 3-roller/belt fixing system has consecutive or non-consecutive multiple nips, and thus the toner that melted in the first nip is secondarily fixed by a pressure applied in the second nip. Such a multiple fixing system is beneficial in that an initial unnecessary pressure is removed as compared to that of a single fixing system, but during a return from the first nip to the second nip, a stable transfer of the image and paper separability must be attained beforehand. To be suitable for such a system, a toner having a sufficient melting property at a first pressure-applying unit, and sufficient paper separability (anti-wrap jam) and a stable image (anti-blurring) at a second pressure-applying unit is necessary.